1. Field of the Invention
The present invention relates to an optical recording medium.
2. Description of the Related Art
FIG. 1 is a cross-sectional view showing a compact disk which is a conventional optical recording medium, e.g., a read-only memory (ROM) type optical disk. The optical disk has a transparent substrate 1 formed of, for example, a polycarbonate (hereinafter referred to as PC) as a comparatively thick glass or plastic having a thickness of 1.2 mm. On one surface of the substrate 1, a minute uneven portion 2 such as a phase recording pit corresponding to, for example, data information, a tracking pregroove or the like is formed by injection molding, for example. A reflective film 3 made of metal such as Al, for example, is deposited on the minute uneven portion 2 by vacuum evaporation or sputtering. Thus, an information recording layer is formed. A protective film 4 generally formed of ultraviolet curing resin as a light curing resin having a thickness of, for example, 0.01 mm which is considerably thin as compared with the substrate 1 is deposited on the reflective film 3 of the information recording layer.
The optical disk having such arrangement has the substrate 1 which is a light transparent layer. Specifically, when information is optically read from the information recording layer of the optical disc, rays of light, i.e., rays of laser light L are irradiated from the side of the transparent substrate 1 having large thickness on the information recording layer to read the information by utilizing optical interference generated by the minute uneven portion 2 thereof.
The reason for reading the information by irradiating rays of light from the thick substrate 1 side as described above lies in the consideration that when there is dust, scratch or the like on the disk surface, the larger a distance between the information recording layer and a surface on which a reading light is incident to a certain extent, the smaller the influence of the dust, scratch or the like on the irradiated rays of light, i.e., the influence of noise of a signal becomes. However, since an amount of coma of a reading signal produced by inclination of the optical disk, i.e., so-called skew is in proportion to thickness of a light transmitting layer in this case, the above-described irradiation of rays of laser light from the side of the comparatively thick and transparent substrate 1 leads to a large amount of coma.
It is possible to exclude noises resulting from the dust, scratch or the like on the surface of the optical recording medium such as the above-mentioned optical disk or the like by a signal processing. Accordingly, as shown in FIG. 2, when the protective film 4 is formed as a transparent film and the rays of light, i.e., laser light are irradiated from the side of the thin protective film (transparent film) 4 to read out information from the minute uneven portion 2, it is possible to reduce the generation of coma. Reduction of the coma leads to a wider skew margin and also to higher recording density.
Even if in the optical recording medium such as the above-mentioned optical disk or the like the rays of light are irradiated from the side of the thin protective film 4 to read or record information from or on the optical disk with the noises resulting from the dust, scratch or the like being removed, then practical improvement with respect to the noise is not sufficient.
The inventor of this application found that one of main causes for producing the noises lay in the reflective film. Specifically, as shown in FIG. 3 which is a schematic diagram of the reflective film 3, the reflective film 3 is formed by vacuum evaporation, sputtering or the like on the minute uneven portion 2 formed by injection molding or the like as described above. The reflective film 3 has on a substrate 1 side a surface which, when the minute uneven portion 2 is seen from the substrate 1 side, is comparatively satisfactory in view of optics because of a condition of the molded surface of the substrate 1. On the other hand, the reflective film 3 has on the opposite side a surface which becomes an irregular surface because Al particles, for example, are irregularly accumulated thereon upon the process of vacuum evaporation, sputtering thereof or the like. Accordingly, when the information is read out from the information recording layer by irradiating rays of light on the above irregular surface, this irregular surface is not sufficiently flat for a wavelength (ranging from 630 nm to 800 nm) of the light irradiated thereon, e.g., a semiconductor laser light L. In particular, an Al evaporated film formed by vacuum evaporation has an Al particle size ranging from 50 nm to 100 nm, which largely influences the shape of the minute uneven portion 2 having a depth of about 100 nm. The Al evaporated film is formed of accumulated Al particles which are transferred thereto with a low energy in the order of 10.sup.-1, and hence has an unsatisfactory orientation of the Al particles and provides the scattering of a large amount of light on the film surface, which leads to generation of a considerable noise.